1. Field of the Invention
The present invention relates in general to a novel method for the determination of biochemical oxygen demand in aqueous liquids containing organic matter and an apparatus for the same, and more particularly to improvements in quickness and accuracy with the method and the apparatus.
2. Description of the Prior Art
Generally, biochemical oxygen demand (hereinafter "BOD") of an aqueous liquid contaminated with organic matter, which is defined as an amount of dissolved oxygen necessary to the metabolic activity of the microorganisms that oxidize the biologically decomposable organic matter to grow, is used as an index that reflects the state of water contamination. BOD is of great importance in industrial working locations. For example, when carrying out a biological process for waste water treatment, the BOD values of influent and in the aeration basin are important parameters in operating the process normally, and in preventing a variety of problems. In addition, since the BOD of effluent is regulated below a legal limit, waste manager always measures the value of BOD to control the effluent quality.
While several quick methods were suggested for the determination of BOD, it is still common practice to measure BOD by Standard Method No. 219 of the American Public Health Association, or by the very similar method of Japanese Industrial Standard JIS K0102-1974. Both methods, however, require a test solution to be held under prescribed conditions for five days before the result of the test can be known. Particularly, the five days is based on the fact that it takes a long time for aerobic microorganisms to grow after a sample is added into a dilute aqueous liquid saturated with oxygen. Thus, BOD measured by the conventional method, in which five days is required to perform the test, is represented as BOD.sub.5. What is worse, the conventional method includes another problem in that the value of the test results depends on the skill of the operator. This is because the conventional test is very complicated and intricate. Hence, the above conventional methods are not convenient to control effluent from industrial plants and the like, which requires quickness and accuracy.
One prior art proposal for reducing the test period to, for example, 30 minutes, was disclosed in U.S. Pat. No. 4,350,763 to Shuichi Suzuki at el. issued on 1982 (hereinafter "763' patent"). This method suggests that a sample solution comes into contact with elementary oxygen and with immobilized microorganisms capable of aerobically metabolizing organic matter in an aqueous liquid, thereby consuming the oxygen. In the 763' patent, an adequate number of microorganisms are immobilized to establish the oxygen-consuming ability of the immobilized cells when in contact with solutions having known BOD. The rate of oxygen consumption of the same microorganisms in contact with an unknown test sample is compared with a calibration chart derived from tests on known standard samples. That is, the 763' patent makes use of the principle that the rate of oxygen consumption is directly proportional to the BOD, so that a plot of BOD vs rate of oxygen consumption is represented by a straight line in a system of Cartesian coordinates. For example, microorganisms which are immobilized in a membrane connected with an electrode come into contact with oxygen-saturated buffer free of oxidizable organic matter, in order to establish a constant current indicative of a reference dissolved oxygen (hereinafter "DO"). And when the oxygen-sensitive electrode is immersed in waste water or the like which contains molecular oxygen, the oxygen content of this sample solution is sensed by the electrode. As the immobilized microorganisms metabolize the organic material in the waste water, a resulting variation in the output current of the electrode is obtained. The method utilizing the oxygen-sensitive electrode proposed in 763' patent is advantageous in many aspects, such as quickness, simplicity, and accuracy, as compared with the conventional BOD.sub.5.
However, the method of the 763' patent has disadvantages as follows. First, if the number of the microorganisms immobilized in the membrane is not constant, the rates of oxygen consumption are different in respective test case for even the same waste water. Hence wrong BOD results are obtained. Second, a standard curve is necessary to plot a calibration chart, which results from indirect determination of BOD, so that another standard curve is drawn again, depending on the activity variation of the microorganisms. Third, if some constituents of the organic matter do not pass through a general bio-degradation pathway, BOD values thereof are impossible to measure, or may be lower than the real values.